In non-volatile memory devices, unit cells of the device maintain data stored in the unit cells even when a power supply is cut off. Widely used non-volatile memory devices include flash memory devices. Unit cells of conventional flash memory devices, typically, have an electrically insulated floating gate. Depending on whether the electrons in the floating gate exist or not (or a variation of an amount of the electrons), the data stored in the flash memory cell may be detected as logical “1” or logical “0” values.
The flash memory cell typically uses a high operation voltage (i.e. a program voltage or an erase voltage) to inject the electrons into the floating gate or to extract the electrons from the floating gate. Therefore, conventional flash memory devices typically use a peripheral circuit for controlling the high operation voltage. Furthermore, fabrication of conventional flash memory devices may be complicated. In addition, power dissipation of the flash memory device may increase as a result of the high operation voltage.
A phase change memory device has been proposed as a non-volatile memory device. The phase change memory device uses phase change material to store data. The phase change material, typically, has an amorphous state and a crystalline state. The phase change material in the amorphous state, typically, has a resistivity higher than that of the phase change material in the crystalline state. Therefore, the logic information stored in a unit cell of the phase change memory device may be determined by sensing the current flowing through the phase change material. Widely-known phase change materials include GST (or Ge—Te—Sb) which is a compound including germanium Ge, tellurium Te and stibium Sb.
Typically, the phase change material is converted into the amorphous state and the crystalline state by heat. Specifically, if heat close to a melting point of the phase change material is supplied to the phase change material and then the phase change material is cooled rapidly, the phase change material is converted to the amorphous state. In contrast, if heat corresponding to a crystallizing temperature lower than the melting point is supplied to the phase change material for a long time and then the phase change material is cooled, the phase change material is converted to a crystalline state. For example, if the GST is supplied with heat to approximately a melting point (about 610° C.) and then cooled rapidly (for about 1 ns), the GST is converted to an amorphous state. If the GST is supplied with heat to the crystallizing temperature (about 450° C.) for a relatively long time (30˜50 ns) and then cooled, the GST is converted to a crystalline state.
Conventionally, the heat supplied for conversion of the phase change material is Joule's heat. That is, the current flowing through the phase change material is used to generate Joule's heat, such that the phase change material is heated.
One example of a phase change memory cell is disclosed in U.S. Pat. No. 5,933,365 by Patrick Klersy et al. FIG. 1 is a cross-sectional view illustrating such a conventional phase change memory device. Referring to FIG. 1, a first heating layer 3 is disposed on a first dielectric layer 1, and a first electrical contact layer 2 is interposed between a portion of the first heating layer 3 and the first dielectric layer 1. The second dielectric layer 4 covers the first dielectric layer 3. A contact hole 5 is formed to penetrate the second dielectric layer 4 and to expose a predetermined region of the first heating layer 3. A phase change material layer 6 contacts the first heating layer 3 through the contact hole 5 and is disposed on the second dielectric layer 4. A second heating layer 7 and a second electrical contact layer 8 are sequentially stacked on the phase change material layer 6. The contact area of the phase change material layer 6 and the first heating layer 3 is identical to an area of the first heating layer 3 exposed in the contact hole 5.
An amount of current flowing through the contact hole 5 (i.e., a contact surface of the first heating layer 3 and the phase change material layer 6) is controlled to convert a portion of the phase change layer 6 neighboring the contact surface into an amorphous state or a crystalline state.
In a conventional phase change memory cell, the amount of the operation current for converting the phase change material layer 6 into an amorphous state or a crystalline state depends on the area of the contact surface of the phase change material layer 6 and the first heating layer 3. That is, as a width W0 of the contact hole 5 related to an area of the contact surface decreases, a density of current flowing through the contract hole 5 increases. It is well known to those skilled in the art that the Joule's heat increases in proportion to the current density. As a result, as a width W0 of the contact hole 5 decreases, the amount of operation current decreases. Conventionally, a width W0 of the contact hole 5 depends on a photolithographic pattern defined by the photolithographic process, such that a minimum width of the contact hole 5 typically depends on the minimum width limitation of the photolithographic process.